Conventionally, a plurality of winding manners, such as follows, for winding heating tube around a water tank may be adopted:
1. A single condensing tube may be wound around the outer wall of the water tank downwardly;
2. A single condensing tube may be wound around the outer wall of the water tank upwardly;
3. At least two condensing tubes may be wound around the outer wall of the water tank downwardly; and
4. At least two condensing tubes are wound around the outer wall of the water tank upwardly.
When the first and third winding manners may be used, because the refrigerant and water with high temperature are both inside the upper part of the water tank, the temperature of water inside of the upper part may be higher than that inside the lower part of the water tank, thus the heating efficiency may be lowered. Further, with the first winding manner, a single long condensing tube may result in high pressure drop of the refrigerant.
When the second and fourth winding manners may be used, the vapor lock phenomenon of the refrigerant may appear. To be specific, after the water in the water tank may be heated, the water temperature inside the upper part may be higher than that inside of the lower part of the water tank, when cold water may flow therein. Thus, the refrigerant in the condensing tubes wound around the upper part of the water tank may be transformed into gaseous state with high temperature and pressure, after heat transferring with the water with high temperature. And the refrigerant in the condensing tubes wound around the lower part of the water tank may be transformed into liquid state with low temperature and pressure after heat exchanging with the water with low temperature.
In this case, if the pressure differences and flow resistance cannot be overcome by the refrigerant, the refrigerant may not be circulated, and at this time, the suction pressure of the compressor may be very low, resulting higher ratio of high/low pressure of the compressor, even beyond the allowable working range thereof, thus influencing the working life of the compressor and the heat pump water heater accordingly. Meanwhile, since the refrigerant in the condensing tubes may not be circulated or may hardly be circulated, the heating efficiency of the heat pump system may be very low. And at the same time, the compressor may be running in a power-consuming mode whereas the water temperature of the water in the water tank may rise very slowly, thus leading to a deteriorated heating efficiency.
In addition, two temperature sensors are used in a water tank conventionally, one of which is disposed at the upper part of the water tank, and the other one at the lower part of the water tank. Therefore, the lower temperature sensor may detect the temperature drop of the water in the water tank more immediately, when the cold water was injected into the water tank. While the upper temperature sensor is used to control the heating of the water heater, or show the temperature of the usable water. In this case, if only one temperature is used and disposed at the lower part of the water tank, the water temperature detected may be dropped immediately when little cold water was injected into the water tank. Consequently, the actual water temperature may be not shown to the user. Further, the water temperature at the upper part of the water tank is unknown when stopping heating because of the single lower temperature sensor, so that the time of stopping heating of the water heater may be uncontrollable, thus possibly making the water temperature at which the heater is shut down higher than the predetermined temperature. On the other hand, if only one temperature sensor is disposed at the upper part of the water tank, the temperature may be detected only when the cold water is injected enough up to the height of the temperature sensor. Consequently, the time of starting the heater may be later than a predetermined time, and the user have to wait a longer time to use the hot water again.